Gas turbine engines generally include a compressor section, a combustion section and a turbine section. A compressor within the compressor section and a turbine within the turbine section each can include a plurality of rotors attached to a central rotating shaft. The rotors include a plurality of blades that project radially therefrom. Both the compressor and turbine are encased in separate cylindrical housings, creating a clearance between the rotating blades and each housing. The clearance between the blades and the housings should be designed in order to assure that the blades, when rotating, do not make contact with the housings. Because the coefficient of expansion of the blades may differ from that of the housings, the clearance between the blades and the housing must be sufficiently large to accommodate the expanded rotating blades at higher temperatures and loads. Thus, at lower temperatures and loads, the clearance between the blades and the housings may be excessively large. The large clearance will decrease the efficiency of the turbine engine by allowing the gases and/or compressed air to flow around the blades at lower temperatures and loads.
Abradable coatings have often been used to increase the efficiency of turbine engines by limiting the flow area around the turbine and compressor blades. The clearance between the blades and the housings can be reduced by applying the abradable coating to an inner surface of the housings. The clearance can be designed such that when the turbine engine is operating at higher temperatures and loads during a break in period, the outer tips of the blades will make contact with the abradable coating of the housings. The contact will cause the abradable coating to abrade away until there is no contact between the outer tips of the blades and the housings. Thus, the clearance between the blades and the housings will be only as large as necessary to accommodate the blades at high temperatures, thereby increasing efficiency by limiting the air flow around the turbine and/or compressor blades, even at lower temperatures and loads.
Although the abradable coatings have been able to increase turbine engine efficiency, an abradable coating that can be applied to turbine blades operating in low to mid temperature turbine stages of a land-based turbine engine has remained elusive. Often, turbines include rows, or stages, of rotating turbine blades that are subjected to different temperatures. For example, a low temperature stage of the turbine may be subjected to temperatures between 950–1300° F., a mid-temperature stage of the turbine may be subjected to temperatures between 1300–1500° F., and a high temperature stage of the turbine may be subjected to temperatures above 1500° F. Because at each stage, the turbine operates within a different temperature range, an abradable coating that works in one stage of the turbine may not provide the oxidation resistance and structural integrity required for a higher temperature stage.
It is known in the art that abradable coatings, such as the coating, described in U.S. Pat. No. 5,434,210 issued to Rangaswamy et al. on Jul. 18, 1995, can be made from a metallic matrix, a thermoplastic and a dry lubricant. However, it is still unknown what percentages of the matrix-forming component, the solid lubricant and the thermoplastic that can provide abradability while maintaining structural integrity and oxidation resistance at temperatures between 950–1500° F. for extended periods of time. For instance, 80%, by volume, of an abradable coating used for aircraft turbine applications and manufactured by Sulzer Metco includes agglomerates of a thermoplastic (polyester) and a dry lubricant (hexagonal boron nitride). Although the agglomerates supposedly provide abradability, the high volume of the agglomerates likely negatively affect the lifetime and integrity of the coating. Because stationary gas turbine engines often require an overhaul lifetime on the order of 30,000 hours, the Sulzer Metco abradable coating would not be durable in stationary gas turbine engines.
The present disclosure is directed at overcoming one or more of the problems set forth above.